HttpClient single instance with different authentication headers

Question

Given that the .net HttpClient has been designed with reuse in mind and is intended to be long lived and memory leaks have been reported in short lived instances. What guide lines are there where you want to make restful calls to a given endpoint using different bearer tokens (or any authorization header) when calling the endpoint for multiple users?

private void CallEndpoint(string resourceId, string bearerToken) {
  httpClient.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("bearer", bearerToken);
  var response = await httpClient.GetAsync($"resource/{resourceid}");
}

Given the above code could be called by any number of threads on a web application it is easily possible that the header set in the first line is not the same one that is used when calling the resource.

Without causing contention using locks and maintaining a stateless web application what is the recommended approach to creating and disposing HttpClients for a single endpoint (My current practice is to create a single client per endpoint)?

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Lifecycle

Although HttpClient does indirectly implement the IDisposable interface, the recommended usage of HttpClient is not to dispose of it after every request. The HttpClient object is intended to live for as long as your application needs to make HTTP requests. Having an object exist across multiple requests enables a place for setting DefaultRequestHeaders and prevents you from having to respecify things like CredentialCache and CookieContainer on every request, as was necessary with HttpWebRequest.

Answer 1

If your headers are usually going to be the same then you can set the DefaultRequestHeaders. But you don't need to use that property to specify headers. As you've determined, that just wouldn't work if you're going to have multiple threads using the same client. Changes to the default headers made on one thread would impact requests sent on other threads.

Although you can set default headers on the client and apply them to each request, the headers are really properties of the request. So when the headers are specific to a request, you would just add them to the request.

request.Headers.Authorization = new AuthenticationHeaderValue("bearer", bearerToken);

That means you can't use the simplified methods that don't involve creating an HttpRequest. You'll need to use

public Task<HttpResponseMessage> SendAsync(HttpRequestMessage request)

documented here.

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Some have found it helpful to use extension methods to isolate the code that updates the headers from the rest of a method.

Example of GET and POST methods done through an extension method that allow you to manipulate the request header and more of the HttpRequestMessage before it is sent:

public static Task<HttpResponseMessage> GetAsync
    (this HttpClient httpClient, string uri, Action<HttpRequestMessage> preAction)
{
    var httpRequestMessage = new HttpRequestMessage(HttpMethod.Get, uri);

    preAction(httpRequestMessage);

    return httpClient.SendAsync(httpRequestMessage);
}

public static Task<HttpResponseMessage> PostAsJsonAsync<T>
    (this HttpClient httpClient, string uri, T value, Action<HttpRequestMessage> preAction)
{
    var httpRequestMessage = new HttpRequestMessage(HttpMethod.Post, uri)
    {
        Content = new ObjectContent<T>
            (value, new JsonMediaTypeFormatter(), (MediaTypeHeaderValue)null)
    };
    preAction(httpRequestMessage);

    return httpClient.SendAsync(httpRequestMessage);
}

These could then be used like the following:

var response = await httpClient.GetAsync("token",
    x => x.Headers.Authorization = new AuthenticationHeaderValue("basic", clientSecret));